1. Field of the Invention
The present invention relates to aluminium nitride ceramics having improved heat-radiation property which can be used as a variety of electronics parts including a substrate for integrated circuits and packaging material.
2. Description of the Related Art
Aluminium nitride ceramic (or AlN ceramics, hereinafter) or a sintered article of aluminium nitride is obtained by sintering a powder material consisting mainly of aluminum nitride. This aluminium nitride ceramic possesses good insulation property, high mechanical strength and high thermal conductivity and is easy to be bonded to metal conductors and hence is expected to be used as a substrate for integrated circuits and packaging material.
When the AlN ceramic is used as a substrate for integrated circuits and packaging material, a metallized layer is usually deposited on a surface of the AlN ceramic, because electronic components such as transistor, diode, IC and LSI can not be mounted directly on the AlN ceramic. Such metallized layer is required also when a circuit pattern is printed on the surface of AlN ceramic.
In order to improve the thermal conductivity of AlN ceramics, it is a usual practice and an effective technique to grow crystalline particles of AlN so that grain boundaries are reduced or eliminated. However, a sintered article of AIN whose crystal particles are grown often possess voids or defects caused by lack of crystal particles, although its thermal conductivity is improved. A metallized layer deposited on such a defective surface of sintered article of AlN does not possess enough strength at an interface between the metallized layer and the sintered article of AlN and will be a cause of such troubles as breakage in circuit patterns and increment of resistivity in wiring lines.
Since two requirements to improve thermal conductivity and to improve surface smoothness are contradictory or trade-off, it is impossible to produce AIN ceramics possessing high thermal conductivity as well as high surface smoothness simultaneously by prior arts.
An object of the present invention is to solve the problems of known techniques and to provide AlN ceramics possessing high thermal conductivity as well as high surface smoothness and methods for producing the same.
The present inventors found that the above-mentioned contradictory two requirements to improve thermal conductivity and to improve surface smoothness can be satisfied simultaneously by depositing a dense smooth surface layer on a surface of a sintered article of AlN.
The present invention provides a aluminum nitride ceramic comprising a sintered article consisting mainly of aluminium nitride and having a thermal conductivity higher than 100 W/m.K at room temperature and a smooth dense surface layer formed on a surface of the sintered article and having a surface roughness (Ra) of lower than 0.3 xcexcm, no defect larger than 25 xcexcm being found on a surface of the surface layer.